In recent years, fuel cells being actively developed in consideration of environmental issues. Various kinds of fuel cells have been known and are classified depending on the kind of the electrolyte used into a polymer electrolyte type, a phosphoric acid type, a fused carbonate salt type, a solid oxide type and the like. Among these, a polymer electrolyte fuel cell (PEFC) is developed for such purposes as a small-size power supply for home use, a portable power supply and a power supply for a movable body, owing to the relatively low reaction temperature and the electrolyte capable of being fixed conveniently, and in particular is actively developed for a purpose relating to automobiles.
PEFC is a fuel cell utilizing such a phenomenon that a polymer membrane having a proton (hydrogen ion) exchange group in the molecule thereof functions as a proton conductive electrolyte when impregnated with water to the saturated state. PEFC is constituted by a polymer electrolyte membrane containing a polymer ion exchange membrane (a cation exchange membrane), and a membrane-electrode assembly (fuel cell) having an anode and a cathode disposed on both sides of the electrolyte, which are held with a separator. A fuel gas, such as hydrogen, supplied to the anode is ionized to hydrogen ion on the catalytic electrode and migrates toward the cathode through the polymer electrolyte membrane, which is appropriately moistened. Electrons generated in that process are taken out to an external circuit and utilized as direct current energy. An oxidant gas, such as oxygen or air, is supplied to the cathode, and the hydrogen ion and the electrons are reacted with oxygen on the cathode to form water.
As the polymer electrolyte membrane, a perfluorosulfonic acid resin membrane (for example, “Nafion”, a trade name, available from DuPont Corporation) has been used, and the cell is generally operated under a temperature condition of approximately 50 to 100° C. for decreasing the resistivity of the polymer electrolyte membrane to provide a high electric power generation efficiency. The polymer electrolyte membrane is demanded to have enhanced electroconductivity and reduced cost, and is difficult to handle since it is a material in an extremely thin film shape.
Accordingly, the polymer electrolyte membrane often gets wrinkled in the outer periphery upon assembling the film with an electrode and upon fabricating a stack by accumulating plural single cells. Furthermore, even in a state free of wrinkles, the polymer electrolyte membrane has a problem since it has the lowest mechanical strength among the constitutional members of the stack. PEFC is permanently tightened up, for example, for preventing from gas leakage and reducing the electric contact resistance among the constitutional member. It has been pointed out that the pressure of the tightening tends to deteriorate particularly the polymer electrolyte membrane with the lapse of time, which brings about reduction in durability of the cell.
JP-A-7-65847 (Patent Document 1) proposes a reinforcing flame that reinforces an electrolyte membrane mechanically and simultaneously prevents a fuel gas and an oxidant gas from leaking through the boundary surface with the electrolyte membrane. The reinforcing flame is preferably those having demanded mechanical strength, corrosion resistance and the like at the operation temperature, and the examples thereof disclosed are polycarbonate, polyethylene terephthalate, a glass fiber-reinforced epoxy resin, titanium and carbon.
JP-A-10-199551 (Patent Document 2) proposes the use of a flame member having gas tightness on an outer periphery of a porous member fixed to both surfaces of an electrolyte membrane. Examples of the material for the flame member disclosed are polycarbonate, a ethylene-propylene copolymer, polyester, modified polyphenylene oxide, polyphenylene sulfide, acrylonitrile-styrene, and the like.
JP-A-2007-103170 (Patent Document 3) proposes a biaxially oriented polyester film using polyethylene naphthalenedicarboxylate (which may be hereinafter referred to as a PEN film) as a reinforcing film for an electrolyte membrane. According to Patent Document 3, the use of the PEN film provides a flame member that has high mechanical strength, excellent thermal dimensional stability at the processing temperature range and the use temperature range, and excellent hydrolysis resistant property under the high humidity use environment.
JP-A-2007-250249 (Patent Document 4) proposes a seal-integrated membrane-electrode assembly used in a fuel cell, in which a reinforcing member having a higher rigidity than the seal member is provided inside the seal member. Examples of the resin for the reinforcing member disclosed are polyethylene naphthalate, polyethylene terephthalate, polyphenylene sulfide, polyether sulfone, polyether ether ketone, polyimide, polypropylene and polyimide.
As described above, various resins have been investigated mainly from the standpoint of mechanical strength as a reinforcing member for an electrolyte membrane of a polymer electrolyte fuel cell.
The reinforcing member is also being demanded to have long-term reliability, and such a material is demanded that not only has the reinforcing strength in the initial stage as in the conventional materials, but also maintains the mechanical strength over a prolonged period of time even under a high-temperature and high-humidity use environment.    Patent Document 1: JP-A-7-65847    Patent Document 2: JP-A-10-199551    Patent Document 3: JP-A-2007-103170    Patent Document 4: JP-A-2007-250249